The present invention relates to a method and apparatus for classifying air foil or vane elements such as turbine stator vanes and the like and more particularly to such a method and apparatus for accurately and repeatably measuring the open flow area formed by a pair of airfoils or vanes interconnected by end buttresses.
A number of classification gauges have been developed in the past in order to provide an accurate determination of the relative effective open area in the various stages of turbojet engines and the like. Such classification gauges have been particularly directed toward the classification or determination of such values in connection with individual vane elements or vane clusters including a plurality of guide vanes, turbine blades and/or other similar structures. For example, U.S. Pat. No. 3,464,119, issued Sept. 2, 1969 to Elmer L. Griggs, disclosed a precision classification guage for classifying individual guide vanes or the like for turbojet engines in order to facilitate assembly of various stages of the engine according to predetermined values for the effective open areas of the individual vanes. U.S. Pat. No. 3,959,886, issued June 1, 1976, also to Elmer L. Griggs, disclosed a similar classification gauge adapted for classifying or determining the relative effective open area for vane clusters rather than individual vane elements. U.S. Pat. No. 4,024,646, issued May 24, 1977, also to Elmer L. Griggs, disclosed yet another apparatus for simultaneously guaging and aligning movable elements of guide vane assemblies including a plurality of vanes or airfoil elements in order to both classify and adjust the effective open area for the vane assembly. Finally, co-pending U.S. Patent Application Ser. No. 133,949 filed Mar. 25, 1980 and entitled "Classification Instrument" disclosed yet another device including a plurality of indicators for providing classification readings corresponding to a number of different features for an airfoil or vane element. All of the above are assigned to the assignee of the present invention.
Substantial background information is set forth by the above noted references, particularly the first noted patent, and reference may be made to those patents for a more complete understanding of certain aspects of the present invention. Generally, it has been found important to obtain precise classification or determination of the effective open area for airfoils or vanes in a cluster or vane assembly. This value is also commonly referred to as "throat area" and must be precisely established in various stages in machinery such as turbojet engines for their proper and most efficient performance.
The classification gauges described in the above noted references have been found to be particularly suitable for determining proper classification of individual guide vanes, vane clusters and even complete arrays of vanes or similar structures. Generally, the above noted classification gauges function to accurately position within a precision jig one portion of an airfoil, such as its trailing edge, other portions of the airfoil then being precisely measured or monitored in order to classify the effective open area for the individual vanes or vane assemblies. Very generally, the open area for such vanes or vane assemblies may be determined by accurately monitoring the location of the trailing edge for one vane, the convex airfoil surface of an adjacent vane, and the spacing between inside surfaces of end buttresses employed to mount the vanes in the assembly.
However, none of the classification guages disclosed by the above noted references provides a simple and reliable means for providing a direct indication of the open flow area for a pair of interconnected vanes. Using the classification gauges disclosed above, such a value could be obtained, for example, by appropriately combining information from various indicators. However, such a step is undesireable since it introduces complexity and possibly error into measurement of the open flow area.
Accordingly, there has been found to remain a need for a relatively simple and accurate classification gauge for directly providing an indication of the open flow area between a pair of interconnected vanes. In this regard, such vanes are commonly interconnected with one of the vanes being in a relatively leading position and the other vane being in a trailing or following position. With such an arrangement, the open flow area or "throat area" is defined by the trailing edge of the leading vane, a convex portion of an airfoil surface of the following vane opposite the trailing edge of the leading vane and the inside surfaces of the opposite end buttresses. In other words, the length of the open flow area is determined by the average spacing between the end buttresses while the width of the open flow area is determined by the average spacing between the trailing edge of the leading vane and the opposite convex portion of the airfoil surface for the following vane. The value for the overall open flow area defined by the two vanes may thus be determined as the product of the average length between the buttresses times the average width between the vanes.